1) ... the sentence It's ages since I have seen him is perfectly fine? ( informal spoken)
2) ... in spoken and written journalistic style present perfect is occasionally used to stress the current importance of events, even if definite past time adverbials are used? This usage is restricted and uncommon.
We've lost so much of our manufacturing industry in the 1980s and early 1990s.( The speaker is speaking in 1998, i.e not the early 1990s)
3) ... you can use present continuous with verbs normally not associated with it?
I'm not wanting to force that idea, but would it be an idea if all Europeans one second European language, do you think? (indirectness)
I'm wondering what that might be. ( more polite than I wonder)
4) ... if a sentence contains an adverbial expressing habits or regular events, it doesn't necessarily mean you have to use the present simple tense?
If the emphasis is on an ongoing process, or what is happening at given, specific time, present continues may be more appropriate.
Their numbers build up on the River Exe over autumn until by mid-winter some 6000 birds are regularly using the roost.
5) ... you can use either present continuous or present simple, without any major difference in meaning, if you refer to psychical feelings experienced at the moment of speaking?
I feel dizzy or I'm feeling dizzy
My back hurts or My back is hurting
more to come...
maybe you know something interesting and want to share it with us? ;)